Abstract

We present a polarimetric technique to improve fundus images that notably simplifies and extends a previous procedure
[Opt. Lett. 27, 830 (2002)]
. A generator of varying polarization states was incorporated into the illumination path of a confocal scanning laser ophthalmoscope. A series of four images, corresponding to independent incoming polarization states, were recorded. From these images, the spatially resolved elements of the top row of the Mueller matrix were computed. From these elements, images with the highest and lowest quality (according to different image quality metrics) were constructed, some of which provided improved visualization of fundus structures of clinical importance (vessels and optic nerve head). The metric values were better for these constructed images than for the initially recorded images and better than averaged images. Entropy is the metric that is most sensitive to differences in the image quality. Improved visualization of features could aid in the detection, localization, and tracking of ocular disease and may be applicable in other biomedical imaging.

Polarimetric CSLO images for subject 2 (upper row). Each image (an average of eight frames) corresponds to an independent incoming polarization and subtends 5.6°. Symbols at the bottom-left corner of each image indicate the orientation of the fast axis of the rotating QWP with respect to the linear polarizer in the GU (circle, −45°; vertical line, 0°; ellipses, 30° and 60°). Spatially resolved elements of the first row of the Mueller matrix are shown in the bottom row. The gray level code for the Mueller matrix is shown at the bottom.

Elements of the first row of the Mueller matrix (left) and constructed images for a retinal area 7.7° across the ONH of subject 4. (a) Original image (average of eight frames) with the highest SNR, entropy, and acutance; (b) constructed image with maximum SNR; (c) constructed image with maximum entropy; (d) average of 32 frames taken with a QWP orientation of 0°. The gray-level code for the Mueller matrix is shown in the center.

Improvement in both SNR and entropy for blood vessel images of two subjects (subject 2, black bars, images shown in Figs. 3, 4; subject 4, white bars, images shown in Fig. 5). Labels 1, 2, and 3 correspond to (1) results of the improvement given by averaging of 32 frames with linear (vertical) polarization, (2) image for nonpolarized incident light (M00), and (3) constructed image with the maximum metric value, respectively. The improvements are calculated with respect to the polarimetric eight-frame-averaged original image with the highest metric value. The results indicate that improvements in image quality are dependent on information obtained with other than vertically polarized light. Not shown is that the M00 image had metric values higher than those of the initial image (average of eight frames) taken with linearly polarized light.

CSLO images for the ONH of subject 3 corresponding to the average of four frames in each of the four polarization states in the GU (upper panels) and spatially resolved elements of the first row of the Mueller matrix (bottom panels). Each image subtends 7°. Numbers at the bottom-left corner of the upper images indicate the orientation of the fast axis of the rotating QWP with respect to the linear polarizer in the GU. The gray-level code for the Mueller matrix is shown at the bottom. The area inside the square on the first image will be used for later comparisons.

Elements of the first row of the Mueller matrix (left) and constructed images for the subarea of the ONH images marked in Fig. 8. (a) Original image with the highest SNR, (b) constructed image with maximum SNR, (c) constructed image with minimum SNR. Each image subtends 2.5°. Numbers at the bottom are the SNR values. The gray-level code for the Mueller matrix is shown in the center.

Polarimetric CSLO images of the ONH for subject 2 (upper row). Each image corresponds to an eight-frame average for an independent incoming polarization. Symbols at the bottom-left corner of each image indicate the orientation of the fast axis of the rotating QWP with respect to the linear polarizer in the GU. Elements of the first row of the spatially resolved Mueller matrix with the corresponding gray-level code are shown in the bottom row.

Poincaré sphere with the Stokes vectors corresponding to the constructed images with maximum (solid symbols) and minimum (open symbols) SNR values in subject 3 for the whole ONH image (Fig. 9; ∼7° across, circles) and a small area within the ONH (Fig. 10; triangles).

Longitude (2*Azimuth, solid symbols) and latitude (2*Ellipticity, open symbols) on the Poincaré sphere of the Stokes vectors providing the images with maximum and minimum SNR, entropy (E), and acutance (A) for ONH images of subjects 2 (squares) and 3 (circles).